G. Pellati

Bainite morphologies in sintered steels: influence of materials and process conditions

ABSTRACT With the introduction of diffusion-bonded (d.b.) powders, metallographers found localised microstructure changes in sintered steels, and frequent presence of bainite. With the sinter-hardening process, the bainite fraction became a tool for rating the method. In this study, depending on different parameters, differences between bainitic structures of homogenous or not-homogeneous sintered steels have been analysed. When using pre-alloys, upper bainite is present with the same morphology. When using d.b. powders, variations of local composition lead to upper bainite of different morphology, each corresponding to a composition range. Quite high Ni local content gives rise to lower bainite, always mixed with martensite. Higher Ni contents lead to incoercible austenite. Upper bainite presents different morphologies, depending on local Ni and Cu amounts. The increase of sintering temperature leads to fewer morphologies of upper bainite, due to greater amount of diffusion processes. The lower bainite has always a typical morphology, whatever process parameters.